1. Field of Invention
The invention relates to grease compositions and, in particular, to grease compositions for fluid bearings.
2. Discussion of Related Art
Fluid bearings, or fluid dynamic bearings, differ from mechanical bearings in that the bearing surfaces (faces) are separated by a liquid or gas rather than by balls or other mechanical means. The fluid layer supports the inner face and keeps the inner and outer bearing faces from contacting. This hydrodynamic layer can allow one face to rotate in relation to the other while being supported by, but not in contact with, the other face. The fluid provides some resistance when the bearing is rotated and typically a lower viscosity fluid provides less resistance. However, lower viscosity fluids may flow more easily than do higher viscosity fluids, and retaining a layer of adequate thickness between the faces may be more difficult with lower viscosity fluids. Therefore, many bearings include a lubricant, such as an oil, as well as a thickener, such as a soap of a metallic salt. These compositions have been able to provide low levels of resistance to rotation as well as an adequate rate of retention in the bearing.